Sunday, June 10, 2012

The CHESS Method: How Expert Witnesses Can Reduce Bias In Their Testimony

Forensic psychologists should strive to provide opinions that are unbiased and fair. Our job is ultimately one of making sure the higher call of justice is served. That sounds easy enough, and most forensic psychologists think they are offering unbiased information and testimony in every case.

I know firsthand how difficult it can be to actually remain unbiased, though. The pull to help the client, or please the attorney, or win the case is very strong. It is hard to tell attorneys the information I found is going to harm their clients' cases. It is even harder to tell them that the entire basis of their clients' arguments are wrong. I have had to do this with attorneys, and it hurts. I worry that I will never get hired as an expert again, and I end up making significantly less money on the case.

On the other hand, I also worry when my conclusions and opinions mesh with what attorneys want to hear. I constantly ask myself if I have come to valid conclusions that happen to support the clients' arguments or if I am just telling attorneys what they want to hear.

In order to reduce bias and to answer these questions in my mind, I use a decision-making model called the CHESS method. Cheryl Wills, a forensic psychiatrist, first wrote about this method in the Journal of the American Academy of Psychiatry and the Law in 2008. It is a five-step process that helps to reduce bias. Here are the steps I use, with a hypothetical example included:

1. Identify a Claim, or a preliminary opinion (e.g. The defendant has delusions and psychotic hallucinations that led him to think he was supposed to rob a bank and shoot all of the bank tellers, which is what the defense attorney wants to use as an argument for the defendant to be found Not Guilty By Reason of Insanity).

2. Develop a Hierarchy of supporting evidence (The defendant was actively hallucinating at the time of the crime, all of the police reports say he was "out of his mind," he has a long history of arrests for bizarre behavior, and he has been treated at a mental health clinic for psychosis for years).

3. Examine the evidence for weaknesses, or areas of Exposure (The defendant also has a long history of methamphetamine dependence, and he tested positive for meth use at the time of the crime).

4. Studying and revising the claim and supporting evidence (I examine all of the evidence again, placing an emphasis on the possibility that his meth use played a role in the crime).

5. Synthesize a revised opinion (Based on psychological testing, a thorough review of the evidence, and knowledge of the effects of meth, my new opinion is that the defendant's behavior was due to methamphetamine intoxication and not a psychotic disorder. Drug intoxication cannot be used to plead Not Guilty By Reason of Insanity).

In the hypothetical example provided above, I might have come to the conclusion that the defendant's behavior was caused by meth and not by mental illness on my own. But, it would have been more tempting to selectively examine the evidence for data that only supported my original conclusion, thus helping the attorney's case. By forcing myself to look for weaknesses in my original argument, I was able to reduce my bias and come to a more accurate conclusion. If I had used the CHESS method to examine weaknesses in my argument and I still came to the original conclusion, I would have been much more confident that my opinion was unbiased and I was not just telling the attorney what he/she wanted to hear.

Thanks for reading-- Max Wachtel, Ph.D.


Wills, C. (2008). The CHESS Method of Forensic Opinion Formulation: Striving to Checkmate Bias. The Journal of the American Academy of Psychiatry and the Law, 36, pp. 535-540.

NOTE: the above text highlighted in yellow was quoted verbatim from Wills (2008).


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